The present invention relates to a volume control valve for a variable displacement type hydraulic rotary machine suitable for use, for example, as a variable displacement hydraulic pump or motor in a construction machine such as a hydraulic excavator.
Generally, a variable displacement type hydraulic rotary machine is used as a hydraulic pump serving as an oil pressure source or as a hydraulic motor for traveling or rotation, in a construction machine or the like. For example, in case of using a variable displacement type hydraulic rotary machine as a hydraulic motor for traveling, the motor volume is switched between a large volume and a small volume by means of an actuator of variable volume, whereby the hydraulic motor is rotated at a low speed with a high torque when the motor volume is large, while the motor is rotated at a high speed with a low torque when the motor volume is small.
As a volume control unit for a hydraulic motor there is known a self-pressure control type volume control unit which controls the motor volume in accordance with a load pressure acting on a hydraulic motor from the exterior (see, for example, Japanese Patent Laid Open No. SHO57-43002). In the self-pressure control type volume control unit, a load pressure of a hydraulic motor is fed as a pilot pressure to a volume control valve and the position of the volume control valve is switched selectively between a large volume position and a small volume position in accordance with the pilot pressure, thereby selectively controlling pressure oil to be fed to an actuator of variable volume.
In this conventional self-pressure control type volume control unit, the volume control valve is composed of a main change-over valve for supplying pressure oil selectively to an actuator of variable volume and a pilot valve which detects a load pressure of a hydraulic motor and which controls the main change-over valve selectively in accordance with the load pressure.
The main change-over valve is switched between a large volume position and a small volume position in accordance with a pilot pressure (load pressure) which is fed through the pilot valve, allowing a pilot pressure receiving area to be changed between the large and small volume positions to prevent the occurrence of hunching at the time of switching from one to another volume.
More particularly, when the hydraulic motor volume is switched from a small to a large volume, the load pressure tends to decrease, so there is a fear that the motor volume may be switched again from the large to the small volume as the load pressure decreases. The load pressure increases upon switching of the motor volume to the small volume, so that the motor volume is again switched from the small to the large volume. Thus, what is called hunching phenomenon occurs.
According to the foregoing conventional construction, for preventing the occurrence of such a hunching phenomenon, the pilot pressure receiving area of the main change-over valve is varied between the large volume position and the small volume position to impart, for example, such a hysteresis characteristic as shown in FIG. 5 which will be described later to a set value of the pilot pressure (load pressure) for changing the motor volume.
In the above conventional volume control unit, since the volume control valve used in the hydraulic motor volume control unit is composed of the main change-over valve which supplies pressure oil selectively to the actuator of variable volume for the hydraulic motor and the pilot valve which selectively controls the main change-over valve in accordance with a load pressure of the hydraulic motor, it is necessary to provide the pilot valve separately from a spool of the main change-over valve, thus resulting in that the whole of the control unit becomes complicated, causing an obstacle to the attainment of size reduction.
As another conventional example, for example in Japanese Utility Model Laid Open No. SHO62-45401 (Publication No. HEI6-28345) there is disclosed a construction wherein a spool of a volume control valve constituted by a hydraulic pilot valve is formed in a stepped shape and a load pressure of a hydraulic motor is exerted as a pilot pressure on a large-diameter side of the spool, whereby the volume control valve is switched from a small volume position to a large volume position in accordance with the load pressure. In this volume control valve, a pressure oil feed/discharge port formed on a high pressure side is blocked by a land portion of the spool until the load pressure decreases in a predetermined certain range, thereby imparting a hysteresis characteristic to a set value of the pilot pressure.
However, this conventional construction is merely such that when the load pressure begins to decrease at a large motor volume, the pressure oil feed/discharge port formed on the high pressure side is blocked by a land portion of the spool to suppress the decrease of the motor volume. If the pressure oil leaks even a slight amount from the spool land portion, the motor volume switches to the small volume side and thus it is difficult to control the motor volume stably.
As a further conventional example, for example in Japanese Patent Laid Open No. HEI1-116301 there is disclosed a construction wherein, for switching the motor volume between large and small volumes using an external command pressure, a stepped portion for receiving an external command pressure is formed in a spool of a volume control valve, and on this stepped portion side an external command pressure receiving area is varied according to a slide position (drive position) of the spool.
According to this conventional construction, however, a sleeve is fitted on the stepped portion side of the spool and the external command pressure receiving area is varied according to a relative position between the stepped portion of the spool and the sleeve, thus requiring the provision of the sleeve separately from the spool. As a result, the number of components used increases and the structure of the volume control valve becomes complicated; besides, it is difficult to reduce the entire size.
The present invention has been accomplished in view of the problems of the conventional techniques mentioned above and it is an object of the invention to provide a volume control valve for a variable displacement type hydraulic rotary machine capable of reducing the number of components used, thereby improving the assembling work efficiency, capable of forming the whole in a compact shape and thereby attaining the reduction of size, and further capable of stabilizing and automating volume control.
For achieving the above-mentioned object and for controlling the volume of a variable displacement type hydraulic rotary machine with use an actuator of variable volume, the present invention is applied to a volume control valve for the variable displacement type hydraulic rotary machine, the volume control valve being constructed such that a load pressure of the variable displacement type hydraulic rotary machine is received as a pilot pressure to switch pressure oil to be fed to the actuator from one pressure level to another.
In one aspect of the present invention there is provided a volume control valve for a variable displacement type hydraulic rotary machine, comprising a valve housing having a spool sliding bore, with a high pressure port, a tank port, a pilot port, and a pressure oil feed/discharge port for an actuator of variable volume being formed in the spool sliding bore in axially spaced positions; a spool inserted into the spool sliding bore of the valve housing and adapted to slide axially through the spool sliding bore to selectively establish and block communication of the pressure oil feed/discharge port with the high pressure port and the tank port; a first pressure receiving portion formed in the spool to receive a pilot pressure introduced from the pilot port, thereby displacing the spool axially within the spool sliding bore; a bottomed axial bore formed in the spool, the axial bore extending in the axial direction of the spool and being open to an end face of the spool; a piston inserted slidably into the axial bore so as to close the open end of the axial bore, the piston defining an oil chamber between it and the bottom of the axial bore and receiving a hydraulic reaction force induced within the oil chamber; a second pressure receiving portion formed by the bottom of the axial bore and adapted to receive an internal pressure of the oil chamber, thereby changing a total pressure receiving area of the spool in conjunction with the first pressure receiving portion; and an oil passage formed in the spool at a position corresponding to the oil chamber, the oil passage causing the oil chamber to come into communication selectively with ports different in pressure out of the said ports when the spool is put in a sliding displacement within the spool sliding bore.
According to this construction, the oil passage causes the oil chamber to communicate with ports different in pressure, e.g., pilot port and tank port, selectively in accordance with a sliding displacement of the spool. Upon communication of the oil chamber with the pilot port, the spool receives at the second pressure receiving portion a pilot pressure introduced into the oil chamber, while upon communication of the oil chamber with the tank port, the pilot pressure receiving state is cancelled. Thus, the total pressure receiving area of the spool at the first and second pressure receiving portions varies depending on with which port the oil chamber is in communication through the oil passage. By utilizing this change in the pressure receiving area it is possible to impart a hysteresis characteristic to a switching pressure (pilot pressure) of the volume control valve.
In another aspect of the present invention, a biasing means is disposed between the valve housing and the spool to urge the spool constantly in a direction opposite to the pilot pressure receiving direction of the first pressure receiving portion. According to this construction, when the load pressure of the hydraulic rotary machine is low, the spool is put in a sliding displacement in one direction by the biasing means, while upon increase of the pilot pressure the spool is slidingly displaced in the opposite direction against the biasing force of the biasing means by the first pressure receiving portion. During this period, the oil chamber is selectively brought into and out of communication with ports different in pressure, thereby causing the pilot pressure receiving area of the spool at the first and second pressure receiving portions to be varied, whereby it is possible to impart a hysteresis characteristic to the switching pressure of the volume control valve.
In a further aspect of the present invention there is provided a volume control valve for a variable displacement type hydraulic rotary machine, comprising a valve housing having a spool sliding bore, with a high pressure port, a tank port, a pilot port, an external command pressure port, and a pressure oil feed/discharge port for an actuator of variable volume being formed in the spool sliding bore in axially spaced positions; a spool inserted into the spool sliding bore of the valve housing and adapted to slide axially through the spool sliding bore to selectively establish and block communication of the pressure oil feed/discharge port with the high pressure port and the tank port; a command pressure receiving portion formed in the spool to receive an external command pressure introduced from the external command pressure port, thereby displacing the spool in the axial direction; a first pressure receiving portion formed in the spool so as to be axially opposed to the command pressure receiving portion and adapted to receive a pilot pressure introduced from the pilot port, thereby displacing the spool in a direction reverse to the command pressure receiving portion; a bottomed axial bore formed in the spool, the axial bore extending in the axial direction of the spool and being open to an end face of the spool; a piston inserted slidably into the axial bore so as to close the open end of the axial bore, the piston defining an oil chamber between it and the bottom of the axial bore and receiving a hydraulic reaction force induced within the oil chamber; a second pressure receiving portion formed by the bottom of the axial bore to receive an internal pressure of the oil chamber, thereby changing a total pressure receiving area of the spool in conjunction with the first pressure receiving portion; and an oil passage formed in the spool at a position corresponding to the oil chamber, the oil passage causing the oil chamber to come into communication selectively with ports different in pressure out of the said ports when the spool is put in a sliding displacement within the spool sliding bore.
According to this construction, the volume control valve can be selectively controlled using the external command pressure. For example, in a lowered state of the external command pressure down to the tank pressure level, it is possible to maintain the spool at a slide position thereof in one direction irrespective of the pilot pressure introduced from the pilot port and fix the volume control valve at a large volume position for example. When the external command pressure is increased to displace the spool in the opposite direction, the spool becomes slidable in one direction or in the opposite direction in accordance with a load pressure (pilot pressure) of the hydraulic rotary machine. In this state, when pushed in one direction upon receipt of a pilot pressure, the spool receives an external command pressure in the opposite direction. Thus, the volume control valve can make a selective volume control by utilizing a difference between the external command pressure and the pilot pressure, i.e., difference between pressure receiving areas, and with a hysteresis characteristic for the pilot pressure.
According to the present invention, the command pressure receiving portion is formed by the end face at one end of the spool. Consequently, the external command pressure can be received by the whole of one end face of the spool and, even if the external command pressure is a relatively low pressure, the command pressure receiving portion can receive the external command pressure at a large pressure receiving area.
In a still further aspect of the present invention, a biasing means is disposed between the valve housing and the spool to urge the spool constantly in a direction opposite to the external command pressure receiving direction of the external command pressure receiving portion.
With this construction, for example when the external command pressure has been decreased down to the tank pressure level, by urging the spool in one direction with use of the biasing means it is possible to maintain the spool at a slide position thereof in one direction irrespective the pilot pressure introduced from the pilot port and fix the volume control valve at a large volume position for example. When the external command pressure is set large, the spool can be pushed in the opposite direction against the biasing force of the biasing means and in this state it becomes possible to let the spool slide in accordance with a load pressure of the hydraulic rotary machine, whereby it is made possible to effect a selective volume control with a hysteresis characteristic imparted to the pilot pressure.
In a still further aspect of the present invention, an external command pressure chamber communicating with the external command pressure port is defined between the command pressure receiving portion and the valve housing, and a throttle for generating a damper action in the external command pressure chamber is provided in a command pressure conduit which connects the external command pressure port to an external command pressure supply means.
According to this construction, even in the event of an instantaneous variation of the load pressure, for example at the time of start-up of the hydraulic rotary machine, it is possible to let the external command pressure chamber act as a damper chamber. Thus, not only it is possible to suppress an instantaneous motion of the spool and thereby suppress the occurrence of a hunching phenomenon but also it is possible to stabilize the selective volume control.
In a still further aspect of the present invention, an external command pressure chamber communicating with the external command pressure port is defined between the command pressure receiving portion and the housing, a throttle for generating a damper action in the external command pressure chamber is provided in a command pressure conduit which connects the external command pressure port to an external command pressure supply means, and a seal member for sealing the external command pressure chamber in a liquid-tight manner with respect to the pilot port is disposed between the valve housing and the spool.
Also in this case the external command pressure chamber can be allowed to act as a damper chamber by the throttle provided in the command pressure conduit, whereby not only an instantaneous motion of the spool can be suppressed to suppress the occurrence of a hunching phenomenon but also the selective volume control can be stabilized. Besides, with the seal member, it is possible to prevent a high pressure from leaking from the pilot port to the external command pressure chamber side and hence xe2x80x9cconfined pressurexe2x80x9d is prevented from being developed by the throttle within the external command pressure chamber.
In a still further aspect of the present invention, a pipe joint which constitutes a part of the command pressure conduit is provided in the external command pressure port of the valve housing, and a throttle is provided in the pipe joint. According to this construction, a throttle for allowing the external command pressure chamber to function as a damper chamber can be incorporated within the pipe joint and thus it is no longer required to separately provide a throttle halfway of a piping which constitutes the command pressure conduit.
In a further aspect of the present invention, the oil passage selectively establishes and blocks communication of the oil chamber with the pilot port and the tank port in accordance with a slide position of the spool, and the spool receives the pilot pressure at a large pressure receiving area on both first and second pressure receiving portion sides when the oil chamber communicates with the pilot port through the oil passage, while when the oil chamber communicates with the tank port through the oil passage, the spool receives the pilot pressure at a small pressure receiving area on the first pressure receiving portion side.
According to this construction, while the oil chamber is in communication with the pilot port through the oil passage, the pilot pressure is introduced into the oil chamber, so that, by the second pressure receiving portion, the pilot pressure receiving area of the spool can be increased by an amount corresponding to the pressure receiving area of the oil chamber. Further, when the oil chamber is in communication with the tank port through the oil passage, the internal pressure of the oil chamber drops to the tank pressure level, so that the spool receives the pilot pressure at only the first pressure receiving portion, whereby the pressure receiving area of the spool can be diminished.
In a still further aspect of the present invention, the second pressure receiving portion has a pressure receiving area smaller than that of the first pressure receiving portion, and when the oil chamber comes into communication with the pilot port, the second pressure receiving portion receives the pilot pressure in a direction reverse to the first pressure receiving portion.
According to this construction, the pilot pressure is conducted into the oil chamber while the oil chamber is in communication with the pilot port, whereby the second pressure receiving portion receives the pilot pressure in a direction reverse to the first pressure receiving portion and thus the pilot pressure receiving area of the first pressure receiving portion can be offset and reduced by the pilot pressure receiving area of the second pressure receiving portion. When the oil chamber communicates with the tank port of a low pressure, the internal pressure of the oil chamber drops to a low pressure level and the pressure acting on the spool in a direction reverse to the first pressure receiving portion becomes low, so that the spool can receive the pilot pressure at a large pressure receiving area on the first pressure receiving portion side and it is possible to relatively increase the pilot pressure receiving area.
In a still further aspect of the present invention, the first pressure receiving portion is formed in the spool as a bottomed bore extending in the axial direction of the spool, the bottomed hole having a diameter larger than the diameter of the axial bore and being open to an end face of the spool on the side opposite to the axial bore, and a piston member larger in diameter than the piston is inserted slidably into the bottomed bore to define a pilot pressure receiving chamber which is constantly in communication with the pilot port.
According to this construction, a pilot pressure receiving chamber always communicating with the pilot port can be defined between the bottom portion of the bottomed bore and the piston member and the first pressure receiving portion formed as the bottomed bore within the spool can always receive the pilot pressure through the pressure receiving chamber.
In a still further aspect of the present invention, the oil passage selectively establishes and blocks communication of the oil chamber with the pilot port and the external command pressure port in accordance with a slide position of the spool, and the second pressure receiving portion receives the pilot pressure or the external command pressure in a direction reverse to the first pressure receiving portion.
According to this construction, while the oil chamber is in communication with the pilot port, the pilot pressure is conducted into the oil chamber, whereby the second pressure receiving portion receives the pilot pressure in a direction reverse to the first pressure receiving portion and thus the pilot pressure receiving area of the first pressure receiving portion can be offset and reduced by the pressure receiving area of the second pressure receiving portion. When the oil chamber communicates with the external command pressure port, the internal pressure of the oil chamber can be lowered to the level of the external command pressure, so that the pressure acting on the spool in a direction reverse to the first pressure receiving portion becomes low. Consequently, the spool can receive the pilot pressure at a large pressure receiving area on the first pressure receiving portion side and thus the pilot pressure receiving area can be increased relatively.
In a still further aspect of the present invention, the spool is constituted by a stepped stool whose one end side is larger in diameter than the other portion thereof, and the first pressure receiving portion is formed by a stepped outer periphery portion of the spool which is positioned on the larger-diameter side of the spool.
According to this construction, at the outer periphery on one end side of the spool there can be formed the first pressure receiving portion as an annular portion in the position of the stepped portion having a large diameter, and the spool can be put in a sliding displacement by the pilot pressure acting on the first pressure receiving portion. In this case, since it is not necessary to form a bottomed bore separate from the axial bore in the spool to define a pilot pressure receiving chamber, it is possible to shorten the overall length of the spool.
In a still further aspect of the present invention, the oil passage is formed so that when the oil chamber is communicated with the pilot port, the oil chamber is brought out of communication with the other ports almost simultaneously, while when the oil chamber is communicated with the other ports, the oil chamber is brought out of communication with the pilot port almost simultaneously.
In this way the spool can establish and block communication between the oil chamber and the ports through the oil passage with zero lap. For example, it is possible to prevent the oil chamber from communicating with the pilot port and the tank port or the external command pressure port at a time which would result in the internal pressure of the oil chamber becoming unstable.
In a still further aspect of the present invention, the spool has a plurality of lands for blocking communication between ports different in pressure, and the oil passage has a throttle passage at a position where the oil chamber is brought into and out of communication with a port lower in pressure than the pilot port out of the plural ports.
According to this construction, even when the oil chamber comes into communication with the tank port for example in accordance with a sliding displacement of the spool after the pilot pressure has been introduced into the oil chamber from the pilot port to raise the internal pressure of the oil chamber, the raised pressure in the oil chamber can be prevented by the throttle passage from flowing out as a jet to the tank port side, thus making it possible to prevent the occurrence of such an inconvenience as the generation of an abnormal pressure on the tank port side which is low in pressure.